Essentials: Improve Flexibility with Research-Supported Stretching Protocols

In this Huberman Lab Essentials episode, Andrew Huberman, a professor of neurobiology and ophthalmology at Stanford School of Medicine, revisits the science and practice of flexibility and stretching. His central framing is that getting more flexible is not purely a matter of pulling on muscle: "flexibility and the process of stretching and getting more flexible involves three major components. Neural, meaning of the nervous system, muscular, muscles, and connective tissue." These components are "all kind of weaved together and braided together in complicated ways," so a stretching protocol that works, he argues, has to respect the nervous system as much as the tissue.

The starting point is a fact he wants "everyone" to know: "Your nervous system controls your muscles. It's what gets your muscles to contract." Within the spinal cord, motor neurons release the chemical acetylcholine "onto the muscles," which "causes the muscles to contract." Contracting muscles move limbs "by way of changing the length of the muscle, adjusting the function of connective tissue like tendons and ligaments." Flexibility, in this account, begins as a control problem for the nervous system, not simply a mechanical property of tissue.

Huberman describes two spinal mechanisms that listeners only need to "hold in mind for this episode." The first is the muscle spindle: sensory neurons "that wrap around the muscle fibers sense the stretch of those muscle fibers" and send information "from the muscle back to the spinal cord." The loop closes when a muscle elongates past a safe range: the sensory neurons "send an electrical signal into the spinal cord such that there is an activation of the motor neuron," which then contracts the muscle and brings "the limb back into a safe range of motion." In other words, part of what limits your stretch is a protective reflex, not just tissue stiffness.

The second mechanism senses load. At the end of each muscle sit tendons, and neurons "closely associated with those tendons that are called Golgi tendon organs" detect "how much load is on a given muscle." Under extreme load they fire into the spinal cord and "shut down, not activate, but shut down motor neurons," preventing contraction. Huberman's concrete example is attempting to lift a weight so heavy that "the force that you're generating could potentially rip your muscles or your tendons off of the bone," disrupting joints and tearing ligaments. The Golgi tendon organs, "these GTOs as they're called," act as "a safety mechanism" that makes contraction "impossible."

Crucially, these spinal circuits are not the whole story: "There are also mechanisms that arrive to the neuromuscular system from higher up in the nervous system, from the brain." That sets up the episode's deeper claim — that flexibility training is also brain training.

Huberman introduces a set of neurons he guesses "99.9% of you have never heard of": Van Economo neurons, exceptionally large cells in the posterior insula that "seem uniquely enriched in humans" and "probably perform essential roles in our ability to regulate our physiology and our emotional state." The insula itself, "the main brain area that's associated with interpreting what's going on in our body," supports interoception — sensing "things in our internal world, within our body," as distinct from exteroception, the sensing of the external world.